1. Field of the Invention
The present invention relates to a density processing method in an image forming apparatus such as a copying machine.
2. Description of the Prior Art
In a digital color copying machine, a document is first irradiated by an exposure lamp in a scanner section, and its reflected light is detected by a CCD (Charge Coupled Device) and sequentially converted into an electric signal. In this case, an image of the document is separated for each color and for each pixel by the CCD into an electric signal corresponding to the density of each pixel. This electric signal is sent to an image processing section.
In the image processing section, an output of the CCD is digitally converted and then, the variations of the CCD, the exposure lamp and the like are corrected for each signal having each color (B (blue), G (green) or R (red)) in a shading correcting section. Signals having respective colors (BGR signals) are converted into toner density signals (YMC (yellow, magenta and cyan) signals) in a BGR-YMC converting section. In addition, a BK (black) signal is produced from the YMC signals in a BK producing section.
Thereafter, in the YMC signals and the BK signals, the density levels of the respective colors are corrected in accordance with the characteristics of a filter and toner in a color correcting section. Furthermore, color conversion of a designated color is performed in a color converting section, and processing, such as trimming or masking, is performed in a synthesis processing section. Thereafter, in a density processing section, the level of a digital density signal sent is converted depending on a developing color, the copy density designated in an operating section, the type of document image designated in the operating section, and the like. Thereafter, the digital density signal is sent to a printer section through a variable magnification and movement processing section for performing variable magnification and movement processing in the main scanning direction of an image, to accomplish recording on paper.
Meanwhile, in the digital copying machine or the digital color copying machine, density processing using a dither method has been generally performed in the density processing section so as to obtain a gray scale image. A digital color copying machine already developed by the applicant of the present application is so adapted that characteristic data representing a gray level of a recording pixel relative to a gray level of a reading pixel (hereinafter referred to as input gray level-output gray level characteristic data) have been previously generated using a dither matrix having 2.times.2 pixels as one block and stored in a memory device, to find data representing a gray level of a recording pixel (hereinafter referred to as output gray level data) corresponding to data representing a gray level of a reading pixel (hereinafter referred to as input gray level data) inputted on the basis of the input gray level-output gray level characteristic data. There are 256 gray levels of the reading pixel, and there are 64 gray levels of the recording pixel. A density processing section in this copying machine is shown in FIG. 4.
The density processing section 10 comprises an address generating circuit 101 and a table memory 102 and is controlled by a CPU 20. The CPU 20 comprises a ROM 21 and RAMs 22 and 23. A plurality of types of input gray level-output gray level characteristic data previously generated are stored in the ROM 21 depending on a developing color (M, C, Y or BK), the copy density designated in the operating section, and the type of document image designated in the operating section.
The CPU 20 transfers one type of input gray level-output gray level characteristic data corresponding to the developing color, and the copy density and the type of document image designated in the operating section out of the plurality of types of input gray level-output gray level characteristic data from the data ROM 21 to the table memory 102. Image data representing a gray level of a reading pixel (input gray level data) and a signal representing the position of the reading pixel (hereinafter referred to as a pixel position signal) comprising a line signal HSYNC and a dot signal CLK (not shown) are sent to the address generating circuit 101. The address generating circuit 101 outputs a signal for specifying an address storing output gray level data corresponding to the input gray level data and the pixel position signal sent (a pixel in the dither matrix which corresponds to the reading pixel) out of addresses in the table memory 102. Consequently, the output gray level data stored at the specified address is outputted from the table memory 102.
Input gray level-output gray level characteristics are subtly changed in many cases even if copying machines are of the same type. In order to provide characteristics suitable for all the copying machines, the input gray level-output gray level characteristics must be corrected for each copying machine.